I've made quite a few 3D printed parts from Shapeways (a company in the netherlands).
They use a Nylon-12 (IIRC) material for their plastic, the "strong Flexible" type and can dye it.
Its very cool for making prototypes and for certain parts, for example I made magazine baseplates for my handgun that are extended so I can use a flared mag well. The down side is YES, the material isn't extremely strong, and under certain types of load the feed lips would definitely crack, or something would break on it.
I also made an MP5-SD handguard for my USC-UMP conversion (although I don't use it often) which has a very skeletonized interior to reduce cost mainly, it's still about 1/8th inch thick at most places which is the minimum I would go for anything structural. It still has 2/3rds of the handguard supported by the factory USC receiver.
So while it's a very cool way to make stuff, and some structural parts are feasible to make - Something like a magazine is not, unless you had the space to make thick feed lips. If you wanted to make a single or 1.5x stack magazine then it would possibly work, but it will wear out quite quickly. The fibreglass re-enforced nylon 6-6 or Nylon-6 they use for most gun/magazine parts is an extremely strong plastic for it's weight and volume.
ETA
That is not going to last very long due to the type of plastic. What this would be really useful for is making boots for the LAR mags that makes them the length of 30 rounders. Then they could be used in Service Rifle. Ontario SR, requires you to carry 60 rds. This would cut the number of mags you need to carry in half, while still allowing you to use the mag as a monopod. The rules allow for boots, provided they are no bigger than regular mags. I'm currently machining some out of aluminum...
That would be a perfect type of project for Shapeways. I don't have an LAR 10-rounder (or a pinned 30 rounder) but It would be very easy to make something like that, and for the time and energy probably a very good price too.
If you have a .STL (Stereolithograph) file, you can upload to shapeways with that file, they even have a "dyed black" version of the White-stong-and-flexible although it's a bit light for my tastes, its died deep enough that light scratches won't show.
If you have any measurements for the stuff, I could do some 3D modeling and upload it to my shapeways account and have it listed for anybody to purchase (I don't have a business account so it's just payment to shapeways! FYI). If you're interested shoot me a PM
